1. Field of the Invention
This invention relates to pilot valves for two-stage hydraulic devices and more particularly, although not exclusively, to two-stage proportional hydraulic valves.
2. Description of the Prior Art
One basic problem with known two-stage proportional valves is that the main stage does not always return to null with sufficient speed to avoid a potentially hazardous condition when the electrical supply to the pilot valve is interrupted. This is because the spool of the main stage, assuming it is offset and not in the normal central (null) position when the emergency arises, has to force hydraulic fluid from the pressurised control chamber thereof, through the underlapped metering areas of the pilot valve and thence to tank or reservoir. The underlapped metering areas are, by design, kept small to give good performance under normal operating conditions and this, coupled with the fact that the force of the centring springs for the main stage spool produce a relatively low pressure differential across the underlapped metering areas of the pilot valve spool, means that centring of the main stage spool can take several seconds. If, as a result of the emergency, the main stage has been placed in a hardover position, then serious damage can be caused to the machinery and/or workpiece being controlled by the valve if corrective action is not taken quickly.
It is thus necessary to provide an alternative flow path to discharge the control pressures to tank in the event of an emergency so that fluid under pressure in the main stage can be relieved quickly, whereby the main stage will be nulled or centred quickly.
Solutions to the problem under discussion have been proposed and these have included the provision of an auxiliary dump valve, the provision of an additional spring offset dump position on the pilot spool, and the provision of a spring-separated, split spool which increases metering area size to provide the desired alternative flow path so as to effect rapid release of hydraulic fluid from the main stage control chambers. These previous proposals all have disadvantages: the dump valve involves extra cost and space, and the split spool whilst effective is a little sophisticated and is thus too expensive for inclusion in more basic, cheaper valves.
In U.S. Pat. No. 4,615,358 there is disclosed another solution to the problem in which the spool of a solenoid-operated pilot valve is formed with two passageways each extending internally of the spool between respective end faces of the latter and the periphery of the spool. In normal operation of the valve, the pilot valve control devices seat against the end faces of the spool so as to close the passageways against fluid flow therethrough, the control devices becoming unseated in the event of an interruption in the electrical input to the or each solenoid, whereby both ends of the second stage are at a common pressure so as quickly to relieve the pressure in the second stage of the device, thereby permitting fast return of the latter to the null position. This arrangement has been found to achieve rapid relief of hydraulic fluid from the main stage to allow fast nulling of the latter in a very simple and cost-effective manner but has also been found to suffer the disadvantagve that excessive and constant flow from the pressure port to one or other service port, and thence to tank, via the passageways in the spool can take place if the pilot spool does not centre accurately.